The present invention generally relates to a light emitting diode, and more particularly, to a method of manufacturing, for example, a GaP green light emitting diode to be simultaneously used with a GaAlAs light emitting diode which emits red light.
As shown in FIG. 5(a), a GaAlAs red light emitting diode employing a p-type substrate includes a p-type substrate 3 of GaAlAs, a p-type epitaxial layer 2 of GaAlAs, and an n-type epitaxial layer 1 of GaAlAs.
On the other hand, as illustrated in FIG. 5(b), a GaP green light emitting diode employing an n-type substrate to be used in a light emitting diode for two color emission includes an n-type substrate 4 of GaP, an n-type epitaxial layer 5 of GaP, and a p-type epitaxial layer 6 of GaP.
In the GaP green light emitting diode referred to above, the p-n junction height is far less than the p-n junction height of the GaAlAs red light emitting diode when both light emitting diodes are mounted on mounting surface 7 as shown in FIGS. 5(a) and 5(b). Therefore, it is necessary to increase the p-n junction height of the GaP green light emitting diode (i.e. in the vicinity of the position of the p-n junction of the GaAlAs red light emitting diode) as much as possible for better light deriving effect and simplicity in assembly. For this purpose, the p-type GaP epitaxial layer 6 of the GaP green light emitting diode must be increased in its thickness.
In order to increase the p-n junction height, it is necessary to increase the thickness of the p-type GaP epitaxial layer 6 for the GaP green light emitting diode. The following methods are available:
(1) raising a growth temperature of a liquid phase epitaxial layer during formation of the epitaxial layer based on the liquid phase epitaxial method; PA1 (2) increasing the thickness of the growth solution; PA1 (3) raising the junction forming temperature; and PA1 (4) lowering the cooling temperature.
However, in the conventional liquid phase epitaxial method, there is a problem that, when the growth temperature of the liquid phase epitaxial layer is raised, an amount of nitrogen (N) to be doped at the gaseous phase simultaneously with the growth is undesirably varied. Meanwhile, if the thickness of the growth solution is increased, the flatness of the surface of the epitaxial layer is deteriorated. Furthermore, raising the junction forming temperature or lowering the cooling temperature, results in light emitting diodes which are unsatisfactory from a brightness standpoint.
Recently, a two color emission element has been used as an outdoor light display element which is formed by combining a GaAlAs red light emitting diode using a p-type substrate and a GaP green light emitting diode using a n-type substrate. The GaAlAs red light emitting diode is constituted by a p-n junction (made of GaAlAs epitaxial layers) formed on a p-type GaAlAs substrate, and mounted on a surface 7 with said substrate being disposed next to the mounting surface 7. On the other hand, said GaP green light emitting diode is constituted by a p-n junction (made of GaP epitaxial layers) formed on a n-type GaP substrate, and mounted on the surface with the substrate side being disposed furthest from the mounting surface 7. This configuration is shown in FIGS. 5(a) and 5(b). In this arrangement, the p-n junction height of said GaP green light emitting diode is also located below the p-n junction height for the GaAlAs red light emitting diode. Therefore, it is necessary to raise the p-n junction height of the GaP green light emitting diode as much (i.e. in the vicinity of the p-n junction height of the GaAlAs red light emitting diode) as possible for better light deriving effect and simplicity in assembly.
Therefore, there has been proposed a GaP green light emitting diode as shown in FIG. 1 in which a n-type GaP epitaxial layer 12 and a first p-type epitaxial layer 13 are formed on a GaP substrate 11 and an extra p-type GaP epitaxial layer 14 is formed to raise the p-n junction height of the GaP green light emitting diode such that the p-n junction heights of the GaP and GaAlAs diodes are matched.